The separation of air into its two principal constituents, nitrogen and oxygen, is an important commercial operation which produces several hundred billion cubic feet of each material every year. Where large quantities of either constituent are required, as, for example, oxygen in the manufacture of steel, the large capital costs of cryogenic systems can be justified, and cryogenic procedures are generally employed. For operations with smaller requirements, oxygen and nitrogen can also be produced by pressure-swing adsorption (PSA) processes. In PSA processes compressed air is pumped through a fixed bed of an adsorbent exhibiting an adsorptive preference for one of the main constituents whereby an effluent product stream enhanced in the non-adsorbed (or lesser adsorbed) constituent is obtained. Compared to the cryogenic processes, PSA air separation processes require relatively simple equipment and are relatively easy to maintain. PSA processes, however, have lower product recovery and higher energy consumption than the cryogenic processes. For these reasons, improvements in the adsorption processes remain an important goal. One principal means of improvement is the discovery and development of better adsorbents.